Oxygen-enriched air production employing successive work expansion of effluent nitrogen



Nov. 26, 1968 D. L. SMITH 3,412,567

OXYGEN-ENRICHED AIR PRODUCTION EMPLOYING SUCCESSIVE WORK EXPANSION OF EFFLUENT NITROGEN Filed Sept. 6, 1966 2 Sheets-Sheet l /N l/ENTOR DONALD L. SM/ TH A TTORNE V Nov. 26, 1968 D. L. SMITH 3,412,567

OXYGEN-BNRICHED AIR PRODUCTION EMPLOYING SUCCESSIVE WORK EXPANSION OF EFFLUENT NITROGEN Filed Sept. 6, 1966 2 Sheets-Sheet 2 ENTHALPY,ARB|TRARY UNITS (RELATIVE) I l I DO/VALD L SM/TH A TTORNEV United States Patent 3,412,567 OXYGEN-ENRICHED AIR PRODUCTION EMPLOY- ING SUCCESSIVE WORK EXPANSION 0F EF- FLUENT NITROGEN Donald L. Smith, Berkeley Heights, N.J., assignor to Air Reduction Company, Incorporated, New York, N.Y., a corporation of New York Filed Sept. 6, 1966, Ser. No. 577,421 8 Claims. (Cl. 62-13) ABSTRACT OF THE DISCLOSURE A compressed air stream is initially cooled and then further cooled and partially condensed in the condenser portion of a condenser-vaporizer to form an oxygenenriched liquid and a nitrogen efiluent. The nitrogen effiuent subcools the oxygen enriched liquid in three successive heat exchange steps with the nitrogen efiluent being work expanded after the first and second heat exchange steps. Subcooled oxygen-enriched liquid is expanded into the vaporizer portion of the condenser-vaporizer where it is vaporized. Vaporized oxygen-enriched material as well as heat exchanged nitrogen are used to cool incoming compressed air.

This invention broadly relates to an improved apparatus and process for the production of oxygen-enriched air. More particularly, the invention relates to a low pressure, partial condensation process for the low cost oxygen enrichment of air to supply a product which may be used in industrial processes such as the smelting of iron ore in a blast furnace, or for other purposes.

The use of oxygen as a means of improving the efficiency of some metallurgical and chemical processes has increased to the point where in some instances a single operation consumes the entire output of a large air separation plant. According to forecasts, a major increase in the use of oxygen for this purpose will develop if the cost can be reduced substantially.

Oxygen in its pure form is not generally required or used in these industrial processes. An air feed with an oxygen content around twenty-eight to thirty percent is usually used. Heretofore the air separation plants generally provided a highly refined oxygen product of 95 to 99.5 percent purity to the user who then mixed this product with air to increase the oxygen content of an air feed to the desired concentration.

In general a high purity product can only be obtained at a relatively high cost. The instant invention involves the production of a lower purity product at substantially reduced cost, which can be used to raise the oxygen content of the air feed. In this invention, a process air stream is enriched in oxygen up to about 42 mole percent in a system utilizing a condenser-vaporizer, of the type originally called a dephlegmator, and this product is then supplied to the user, either with or without dilution to adjust the final oxygen content.

The condenser-vaporizer provides indirect heat exchange between a rising stream of saturated compressed air and a descending stream of cold liquid to obtain partial condensation of the air stream while vaporizing the cold ice liquid. The heat of vaporization for evaporating the cold liquid is drawn from the rising air stream and the removal of this heat from the air stream serves to condense the desired portion of the air. The maximum available oxygen content of the condensate at a given pressure is determined by substantial equilibrium at the surface of a pool of the condensate in the bottom of the condenser in contact with the incoming air stream. The condensate at moderate pressures contains about 30 to 50 percent oxygen, which is in the preferred range desired by the user. The process can be adjusted so that the condensate is drawn off and supplied to the vaporizer as the necessary cold liquid, and so that this cold liquid is completely vaporized in condensing the desired fraction of the air stream.

The uncondensed portion of the input process stream remaining after the partial condensation is mainly nitrogen, which is passed out of the top of the condenser as effluent and is used as the principal refrigerant for sustaining the process. A minor portion of the required refrigeration is obtained from the oxygen-rich condensate. Both these sources of refrigeration utilize the power applied by the air compressor to the input air stream.

The percent condensation eifected in the partial condensation process is regulated by the height of the condensing column, the temperature of the cold liquid supplied to the top of the column, the rate of flow of the air stream into the condenser, and the rate of flow of nitrogen efiiuent from the top of the condenser. The resultant percentage of oxygen in the oxygen-enriched output is limited in its maximum amount as above mentioned, by the equilibrium condition between the incoming air stream and the pool of condensate in contact with the air stream at the pressure existing at the surface of the pool.

In order to make heat flow from the incoming air stream to the oxygen-rich liquid condense part of the air while vaporizing the oxygen-rich liquid, it is necessary to raise the saturation temperature of the air by raising the pressure of the air stream in the condenser. For this purpose, it is customary to compress the air stream before cooling it to the saturation temperature. The amount of compression required depends upon the temperature difference needed to promote the desired heat flow. The compression required turns out to be relatively small and consequently less expensive compared with the requirements of air separation plants designed to produce a highly refined oxygen product.

I have found that the refrigeration made available to the required moderate compression of the air stream is, if efficiently utilized, sufiicient not only to reduce the compressed air stream to its relatively high saturation temperature, but also to sub-cool the oxygen-rich liquid the necessary amount so that, when the oxygen-rich liquid is expanded to only slightly above atmospheric pressure, substantially all of it will remain liquid and be at the saturation temperature corresponding to the reduced pressure, thereby providing the temperature differential required in the condenser-vaporizer.

While the dephlegmator has long been known, it has not come into wide use in small installations. It is known that an apparatus of small volume and consequent relatively large surface to volume ratio requires better insulation to maintain a given low temperature than does an apparatus of larger volume and smaller surface to volume ratio. When the dephlegmator was first introduced, the insulation available was much less efficient than now, and there was no call for sufficiently large units that could be cooled to the desired low temperature.

Although the percentage of oxygen obtainable in oxygen-enriched air by means of the condenser-vaporizer is admirably suited to the use of the product in the smelting of iron, or like uses, the process has not been a commercially economical one heretofore because of the lack of a refrigeration system suitably adapted for use with the condenser-vaporizer.

The primary object of this invention is to manufacture oxygen-enriched air by means of a unique process where by production costs are reduced.

Another object of this invention is to utilize a condenser-vaporizer in the production of oxygen-enriched air in a manner not previously known.

A feature of the invention is a specially designed refrigeration system utilizing the moderate compression of the incoming air stream, residing both in the oxygenenriched liquid and in the nitrogen effluent, to bring the incoming air stream down to saturation temperature and to sub-cool the oxygen-enriched liquid to a temperature favorable to the desired partial condensation process.

Other objects, features and advantages will appear from the following more detailed description of an illustrative embodiment of the invention, which will now be given in conjunction with the accompanying drawings, in which:

FIGURE 1 is a flow chart or schematic diagram of an embodiment of the invention; and

FIGURE 2 is a temperature-enthalpy diagram typical of the process illustrated in FIGURE 1.

Referring to FIGURE 1, the entering air is drawn through a dust filter 10, compressed to about 2.4 atmospheres absolute by means of a compressor 11, cooled in an after-cooler 12, and then directed into two regenerators 13, 14 of a set of four regenerators 1346. The air is cooled in the regenerators to approximately saturation temperature and the outgoing products are passed through the remaining regenerators 15, 16. The characteristics of the outgoing products will be more specifically described below, it being sufficient for our purposes at this point only to indicate that oxygen-enriched air and substantially pure gaseous nitrogen are the end products. The details of structure and of operation of the regenerators are Well known in the art, and further description thereof is accordingly unnecessary. It is sufficient to state that the regenerators cool the air from approximately 300 degrees Kelvin to saturation, at about 90 degrees Kelvin. In order to remove water vapor and carbon dioxide from the incoming air, the regenerators are reversed at suitable intervals in known manner by means not shown.

From the regenerators the air, which has been compressed and cooled, is transmitted to the condenser-vaporizer 18 through a conduit 17.

A modification of the classic condenser-vaporizer designed by George Claude and called by him a dephlegmator is illustrated in FIGURE 1. It combines condensing passages 19, constituting a tube bundle, and vaporizing passages 20. The same condenser-vaporizer has been applied to the dephlegmator for obvious reasons. The illustrated device consists of an outer casing 25 having internal end plates 26, 27 which rigidly support the tube bundle 19. As shown, the tubes open into the upper end 30 and lower end 31 of the device. A suitable number of transversely arranged bafiies or trays 32 are mounted in the device between the tubes. The trays control the evaporating liquid flow, preventing the formation of a pool of liquid of uniform composition, as will be more specifically described below.

Cold air at saturation temperature and approximately 2.4 atmospheres absolute pressure is introduced into the condenser at opening 35. This air passes up through the tubes 19 where it is partially condensed and rectified. Substantially pure nitrogen gas leaves the top at 36 and a liquid enriched in oxygen forms a pool 28 in the lower end 31. Approximately 50 percent of the air is condensed to a liquid containing approximately 42 mole percent oxygen. The liquid is drawn off through a conduit 37 and directed into a series of heat exchangers 40, 41, 42. The conduit 36 directs the substantially pure efiluent nitrogen gas into the cold end of the first heat exchanger 40 and this serves to sub-cool the oxygen-enriched liquid through a first stage. The nitrogen gas is then expanded in a turboexpander 50 by which process it drops in temperature and is transmitted to the cold end of the second heat exchanger 41 wherein it further sub-cools the oxygen-enriched liquid. The gas is then further expanded in a turbo-expander 51 and is transmitted to the cold end of the heat exchanger 42 wherein the enriched liquid is further sub-cooled. A conduit 52 then directs the substantially pure nitrogen gas into the regenerator 16 where it is warmed up to ambient temperature.

From the heat exchanger 42 the enriched liquid is conducted to filtering mechanism 55 which may be filled with silica gel or like material adapted to adsorb acetylene and other dangerous hydrocarbons. The liquid is then passed through an expansion or throttle valve 56 and the resulting liquid at approximately 1.3 atmoshperes is fed through a conduit 57 into the top-most tray of the vaporizer portion of the condenser-vaporizer 18. The liquid trickles down from one tray to the next, evaporating while it descends. The liquid acts as a cooling medium about the tubes 19 which contain the rising compressed air. Finally when all the liquid has evaporated, the oxygen-enriched gas emerges at 58. This oxygen-enriched gas is then warmed in the regenerator 15 and supplied to the consumer, with or without dilution with air to adjust the final oxygen content.

Table I shows a set of temperature values at designated points in an illustrative system according to the invention. The figures in the table are based upon an input air pressure to the condenser of 2.4 atmospheres absolute; pressure at outlet of valve 56, 1.3 atmospheres; air flow through compressor 11, 62,600 standard cubic feet per minute; flow of oxygen-rich output, 31,300 s.c.f.m.; flow of nitrogen eflluent, 31,300 s.c.f.m. (50 percent condensation).

TABLE I Temperature, Reference Locatlon degrees point,

Kelvin Figure 2 Input to regencrators 13, 14 300 Input to condenser, conduit 35. 103 Input to Warm end of exchanger 40 89. 8 108 Input to warm end of exchanger 41 88. 7 107 Input to warm end 01' exchanger 42 8G. 3 118 Outlet 01 cold end of exchanger 42 84. 4 123 Outlet of valve 56 83. 3 128 Oxygen-rich vapor, conduit 58 87. 2 130 Oxygen-rich vapor from regenerator 15 299 Nitrogen effluent 8G. 7 101 Input to turbine expander 50- 88. 8 110 Outlet of expander 50 83. 2 Input to expander 51 87. 7 116 Outlet of expander 51 81. 6 121 Outlet of warm end of exchanger 42 85.3 124 Nitrogen vapor from regenerator 16..." 299 In the last column of Table I, the temperature values are keyed to numbered reference points in a typical temperature-enthalpy diagram shown in FIGURE 2.

The refrigeration considerations for the system shown in FIGURE 1 will now be described with reference to Table I and FIGURE 2. As the absolute values of enthalpy involved are of no importance as far as the temperature changes in the refrigeration system are concerned, the abscissae in the diagram are relative, rather than absolute, values of enthalpy. The ordinates are temperatures in degrees Kelvin.

The required pressure in the incoming air stream, 2.4 atmospheres in the example, is determined from the following considerations, together with the usual requirements that there must be sufiicient pressure in the vaporizer to move the vaporized oxygen-rich product through the regenerator 15, and that there must be sufficient pressure in the output of the expander 51 to move the expanded nitrogen efiluent through the heat exchanger 42 and the regenerator 16. In both cases, a typical suitable value for the low pressure is about 1.3 atmospheres.

The expanded oxygen-rich liquid in conduit 57 is poured onto the top tray of the vaporizer at the given low pressure of 1.3 atmospheres in the example. The liquid trickles down from tray to tray, boiling as it goes and changing composition from tray to tray. Entering with composition of about 42 percent oxygen, 58 percent nitrogen, the last drop to evaporate from the bottom tray has a composition of about 72 percent oxygen, 28 percent nitrogen. The boiling point of the mixture varies from about 83.3 degrees at the top of the condenser to about 87.2 degrees at the bottom. These temperatures are represented at the points 128 and 130, respectively, in FIGURE 2, and the warming curve 104 for the vaporizer is substantially a straight line between these two points.

To obtain heat flow from the condenser to the vaporizer, the condenser must be at a higher temperature than the vaporizer at all points along the height of the dephlegmator. In a dephlegmator of suitable design, a temperature difference of about 2.8 degrees at the bottom or Warm end is sufficient. This determines the temperature of the incoming air stream as 90 degrees, represented at the point 103. The cooling curve for the condenser is typically a substantially straight line 102, of somewhat gentler slope than the warming line 104, determining the temperature of the nitrogen efiluent at the top of the condenser as 86.7 degrees, represented at point 101, and giving a temperature differential of about 3.4 degrees at the top of the condenser.

In order to raise the saturation temperature of the incoming air to 90 degrees, the pressure required is about 2.4 atmospheres, as used in the example. Accordingly, the incoming air is compressed to that pressure.

The cooling curve for the oxygen-rich liquid as it passes through the heat exchangers 40, 41, 42 is typically a substantially straight line, as shown at 100. Because the enthalpy scale in the figure is arbitrary, the slope of the line 100 is also arbitrary for present purposes. The warming curve for the nitrogen efiiuent refrigerant in the heat exchangers 40, 41, 42 is typically another substantially straight line, as shown at 106. While the slope of the line 106 in the figure is also arbitrary, the slope of the line 106 is known to be steeper than the slope of the line 100, as indicated in the figure. Since the nitrogen efiluent from the top of the condenser enters the cold end of the heat exchanger 40, represented at point 101, the line 106 must pass through the point 101, as shown.

The temperature, 89.8 degrees, of the oxygen-rich liquid that collects in the bottom of the condenser is slightly cooler than the incoming air. Assuming that a temperature head of one degree is required at the Warm end of the heat exchanger 40, the upper end of the line 106 is determined at the point 110, at the temperature 88.8 degrees. One degree above the point 110, at the same enthalpy value E is the point 108 representing the oxygenrich liquid as it leaves the condenser-vaporizer in conduit 37 and enters the warm end of the heat exchanger 40.

Due to the difference in the slopes of the lines 100 and 106, it is evident that the temperature divergence at the cold end of the heat exchanger 42 would be excessive if an attempt were made to sub-cool the oxygen-rich liquid as much as 5 to degrees without taking special measures. In addition, it would be found that in the usual case the nitrogen effiuent would condense before it could be made sufiiciently cold to be delivered to the cold end of the heat exchanger 42. Thus, the exchangers 40, 41, 42 cannot be combined into a single exchanger, although two might suflice.

Instead, the nitrogen effluent, which comes from the top of the condenser at 86.7 degrees, is delivered to the cold end of the heat exchanger 40, at point 101, and warmed up therein to 88.8 degrees as indicated at point 110. The enthalpy at the point 101 is designated E which value of enthalpy, determining a vertical line intersecting the line at the point 107, determines the cold end temperature, 88.7 degrees, of the heat exchanger 40 and also of the warm end of the heat exchanger 41. The temperature, 87.7 degrees, of the nitrogen effluent leaving the warm end of the heat exchanger 41 is indicated at the point 116, chosen one degree colder than the oxygen-rich liquid entering the exchanger.

The total pressure drop in the turbo-expanders 50 and 51 is preferably divided between the expanders in geometrical proportion, from 2.4 to about 1.77 atmospheres in expander 50 followed by from 1.77 to 1.3 atmospheres in expander 51.

The pressure drop of the nitrogen effluent in the turbine expander 50 from 2.4 atmospheres to about 1.77 atmospheres is indicated by a broken vertical line 112, from point 110, which pressure drop cools the nitrogen efiiuent to about 83.2 degrees at entry into the cold end of heat exchanger 41. The warming line 114 for the nitrogen effluent in heat exchanger 41 is substantially parallel to the line 106. The line 114 has its upper end at the point 116, and its lower end point 115 is determined by the intersection of the line 114 with the temperature line for 83.2 degrees.

The point 115 defines an enthalpy value E The vertical line through this point which intersects the line 100 at the point 118 determines the temperature, about 86.3 degrees, of the oxygen-rich liquid leaving the cold end of the heat exchanger 41 and entering the warm end of the heat exchanger 42. The enthalpy value E also determines the position of the point 124, chosen one degree below the point 118, at temperature 85.3 degrees.

The pressure drop of the nitrogen effiuent in the turbine expander 51 from about 1.77 atmospheres to about 1.3 atmospheres, is indicated by a broken vertical line 120, from point 116, denoting that the nitrogen efiiuent is cooled to about 81.6 degrees. The warming line 122 for the heat exchanger 42 is substantially parallel to lines 106 and 114, and has its upper end at the point 124 and its lower end point 121 determined by the intersection of the line 122 with the temperature line for 81.6 degrees.

The warmed nitrogen efiluent emerging from the warm end of the heat exchanger 42 is delivered to the regenerator 16 for further warming in the process of passing its remaining refrigeration to the incoming air stream, as indicated schematically by an arrow 125.

The point 121 defines the enthalpy value E which in turn determines the temperature, 84.4 degrees, of the oxygen-rich liquid leaving the cold end of the heat exchanger 42, indicated at the point 123. This liquid, being still at approximately 2.4 atmospheres, can be cooled still further, as by throttle expansion in valve 56, as indicated schematically by a broken vertical line 127, to a temperature of 83.3 degrees. This expanded liquid is delivered to the cold end of the vaporizer as indicated at point 128, and is sutficiently cold to effect the desired partial condensation of the air in the condenser. The oxygen-rich liquid that is vaporized in the vaporizer is warmed as indicated by the line 104 to a temperature of about 87.2 degrees as indicated at point and delivered to the regenerator 15 for further warming, as indicated schematically by an arrow 130, the vapor forming the desired product of the illustrated process.

While a working pressure of 2.4 atmospheres absolute in the condenser portion of the condenser-vaporizer has been specified and is the preferred value, a range of pressure from about 2 to 3 atmospheres is allowable, depending upon such factors as the pressure drop in the heat exchangers 13, 14, and the temperature gradient between the heat transfer surfaces of the condenservaporizer. The lower the pressure, the larger the heat exchangers and the condenser-vaporizer required, with the result that the capital investment in plant is increased. On the other hand, the higher the pressure, the more power is required to operate the compressors. Consistent with these general limits, the lowest pressure is preferred.

While a pressure of approximately 1.3 atmospheres absolute has been specified in the vaporizer portion of the condenser-vaporizer, this value is not critical. The pressure should be as low as may be while still sufiicient to propel the products through and out of the system.

The oxygen content of the enriched air product will vary to some extent with the operating pressure of the condenser, ranging from about 43 percent at 2 atmospheres to 41 percent at 3 atmospheres.

Over the entire pressure range it will generally be preferable to adjust the condenser-vaporizer to condense approximately 50 percent of the air stream.

For the expansion of the nitrogen effluent, work expansion as in the turbo-expanders 5t), 51 hasbeen specified rather than valve expansion as in a throttle valve, in order to obtain suiiicient refrigeration to sustain the process. It will generally not be possible to substitute valve expansion for work expansion at this point because such a substitution would require an additional refrigeration unit.

While an illustrative form of apparatus and a method in accordance with the invention has been described and shown herein, it will be understood that numerous changes may be made without departing from the general principles and scope of the invention.

\Vhat is claimed is:

1. A method of producing oxygen-enriched air comprising the steps of compressing an air stream, cooling the compressed air stream substantially to saturation temperature, condensing a portion of the cooled air stream in the condenser portion of a dephlegmator to form oxygen-enriched liquid and a nitrogen efiiuent, withdrawing said oxygen-enriched liquid from said dephlegmator and sub-cooling the same in at least three stages by indirect heat exchange between said oxygen-enriched liquid and the nitrogen efiiuent, work-expanding all of said nitrogen effiuent after it passes through said first stage and then directing said expanded nitrogen efiiuent to said second stage, further work-expanding said nitrogen elfiuent after it passes through said second stage and then directing said further expanded nitrogen efiiuent to said third stage, expanding said liquid following the sub-cooling and directing it into the vaporizer portion of the dephlegmator, utilizing in separate streams both the vapor from the vaporizer portion and the further expanded nitrogen effluent after its passage through the subcooling stages to cool the incoming compressed air, said vapor stream from the vaporizer constituting the oxygenenriched air product.

2. A method according to claim 1 wherein the air stream is initially compressed to about 2 to 4 atmospheres pressure.

3. A method according to claim 2 wherein approximately 50 percent of the incoming air stream is condensed in the condenser portion of the dephlegmator to form an oxygen-enriched liquid of approximately 41 to 43 percent oxygen content.

4. In a system for the production of oxygen-enriched air, in combination, means to compress an incoming air stream, a condenser-vaporizer comprising a condenser portion in indirect heat exchange relationship with a vaporizer portion, said condenser portion being adjusted to condense a part of the incoming air stream and rectify the same to form an oxygen-enriched liquid and a nitrogen effiuent of high purity and said vaporizer portion being adjusted to vaporize substantially the whole oxygen-enriched liquid output of the said condenser portion, means to cool approximately to saturation temperature the out put of said compressor and conduct the same to the said condenser portion of the condenser-vaporizer as a feed stream therefor, means to sub-cool in at least three stages the oxygen-enriched liquid output from said condenser portion, means to expand the said sub-cooled material to a relatively low pressure sufficient to propel the products through the system, means to conduct the said so expanded material to the input of the said vaporizer portion, means to place nitrogen effiuent from said condenser portion into indirect heat exchange relationship with said oxygen-enriched liquid in a first stage of sub-cooling, means to work expand said nitrogen eifiuent leaving said first stage of sub-cooling, means to place said workexpanded material into indirect heat exchange relationship with said oxygen-enriched liquid in a second stage of sub-cooling, means to further work expand said nitrogen efiiuent leaving the said second stage of sub-cooling, and means to place the said further work-expanded material into indirect heat exchange relationship with said oxygenenriched liquid in a third stage of sub-cooling, means conveying the so expanded nitrogen efliuent from said third stage of sub-cooling to said means to cool the incoming air stream to provide a portion of said cooling, and means to warm the said vaporized oxygen-rich material while contributing to the cooling of the incoming air stream.

5. Apparatus according to claim 4 together with means to filter impurities from the sub-cooled oxygen-enriched liquid prior to its expansion to said relatively low pressure.

6. In a system for the production of oxygemenriched air, in combination, means to compress an incoming air stream to a pressure in the range from about 2 to 3 atmospheres absolute, a condenser-vaporizer comprising a condenser portion in indirect heat exchange relationship with a vaporizer portion, said condenser portion being adjusted to condense approximately 5O percent of an incoming approximately saturated air stream and rectify the same to form an oxygen-enriched liquid of approximately 41 to 43 percent oxygen content and a nitrogen effluent of high purity, and said vaporizer portion being adjusted to vaporize substantially the whole oxygen-enriched liquid output of the said condenser portion, means to cool approximately to saturation temperature the output of said compressor and conduct the same to the said condenser portion of the condenser-vaporizer as a feed stream therefor, means to sub-cool in at least three stages the oxygenenriched liquid output from said condenser portion, means to expand the said sub-cooled material to a relatively low pressure sufiicient to propel the products through the system, means to conduct the said so expanded material to the input of the said vaporizer portion, means to place nitrogen effluent from said condenser portion into indirect heat exchange relationship with said oxygen-enriched liquid in a first stage of sub-cooling, means to work expand said nitrogen efiluent leaving said first stage of subcooling, means to place said work-expanded material into indirect heat exchange relationship with said oxygen-enriched liquid in a second stage of sub-cooling, means to work expand said nitrogen efiluent leaving the said second stage of sub-cooling, and means to place the said workexpanded material into indirect heat exchange relationship with said oxygen-enriched liquid in a third stage of subcooling, means conveying the so expanded nitrogen efiiuent to said means to cool the incoming air stream to provide a portion of said cooling, and means to warm the said vaporized oxygen-rich material while contributto the cooling of the incoming air stream.

7. Apparatus according to claim 1, in which the said means to compress an incoming air stream develops a pressure of substantially 2.4 atmospheres absolute in the condenser portion of said condenser-vaporizer and in which the resulting oxygen-enriched liquid has an oxygen content of approximately 42 percent.

8. Apparatus according to claim 1, in which the said pressure of the expanded oxygen-enriched liquid fed into the said vaporizer portion of the condenser-vaporizer is substantially 1.3 atmospheres absolute.

(References on following page) 9 10 References Cited 3,217,502 11/1965 Keith 6239 XR PATENT 3,264,831 8/1966 Jakob 62-13 UNITED STATES S 3,333,434 8/ 1967 Grunbarg et a1 6213 1/1951 Garbo 62-14 fig 5 FOREIGN PATENTS o nson 1 12 195 Fuchs et 1 2 13 861:853 1/1953' y- 8/1964 Koehn et a1. 8/1965 Ruhemann et a1- 62*39 XR NORMAN YUDKOFF, Pnmary Exammer. 10/1965 G b et 1 62 13 XR V. W. PRETKA, Assistant Examiner. 

